Sorry, I think you are missing the point.

Don't even think about not writing down the passphrase if you are going that path. A million things could go wrong. You could lose memory. You could mess up a single character and screw things up (been there, done that). And if you are going to rely on your memory... why not simply remember your seed instead? Simply, don't rely on your memory that way.

If you have a single sig, no passphrase setup, any of your backups falling into the hands of an attacker means you lose your bitcoin.

If you have a single sig + passphrase setup, and you store them together, it's the same thing as the single sig, no passphrase setup.

If you have a single + passphrase setup, and you store the seed and the passphrase in different locations (so that no single location falling into an attacker's hands means losing your bitcoin) you are effectively facing the same problem as a 2-of-2 multisig: you know have two pieces of information (the seed and the passphrase) which are both necessary for recovery, so you have two single points of failure. It's enough to lose one of the two things to lose your bitcoin.

If you have a 2-of-3 multisig, with 3 or more backup locations, there is no single point of failure. You can lose one of the keys and still recover.

So, again: if you compare a simple single sig against a single sig + passphrase or its lookalike, the 2-of-2 multisig, you are increasing security against attackers a bit while increasing security against accidentally losing your bitcoin a lot.